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Re: [dm-devel] Regression in 3.15 on POWER8 with multipath SCSI



On 06/30/2014 12:30 PM, Paul Mackerras wrote:
I have a machine on which 3.15 usually fails to boot, and 3.14 boots
every time.  The machine is a POWER8 2-socket server with 20 cores
(thus 160 CPUs), 128GB of RAM, and 7 SCSI disks connected via a
hardware-RAID-capable adapter which appears as two IPR controllers
which are both connected to each disk.  I am booting from a disk that
has Fedora 20 installed on it.

After over two weeks of bisections, I can finally point to the commits
that cause the problems.  The culprits are:

3e9f1be1 dm mpath: remove process_queued_ios()
e8099177 dm mpath: push back requests instead of queueing
bcccff93 kobject: don't block for each kobject_uevent

The interesting thing is that neither e8099177 nor bcccff93 cause
failures on their own, but with both commits in there are failures
where the system will fail to find /home on some occasions.

With 3e9f1be1 included, the system appears to be prone to a deadlock
condition which typically causes the boot process to hang with this
message showing:

A start job is running for Monitoring of LVM2 mirror...rogress polling

(with a [***     ] thing before it where the asterisks move back and
forth).

If I revert 63d832c3 ("dm mpath: really fix lockdep warning") ,
4cdd2ad7 ("dm mpath: fix lock order inconsistency in
multipath_ioctl"), 3e9f1be1 and bcccff93, in that order, I get a
kernel that will boot every time.  The first two are later commits
that fix some problems with 3e9f1be1 (though not the problems I am
seeing).

Can anyone see any reason why e8099177 and bcccff93 would interfere
with each other?

It might be running afoul with the 'cookie' mechanism.
Device-mapper is using inserting a 'cookie' with the ioctl, and listens to any event containing the cookie to ensure udev has finished processing that device and hence the device node is accessible. Added to this is the problem that we don't have any good means of detecting any changes to device-mapper devices.

EG look at this sequence of events:

add dm-1
remove dm-1
add dm-1

Originally udev would pick up the event, read the details from sysfs, and return control to the kernel.
With bcccff93 udev will _not_have a chance to read the details
from sysfs for 'dm-1', as anything read from sysfs relating to 'dm-1' might infact refer to the _second_ 'add' event, which might be a totally different device.
As far as I know udev doesn't have any mechanism to drop events,
so it'll always process all events. Assuming that the sysfs attributes it reads _do_ relate to that event. If they don't things become interesting ...

(Actually, this issue was always present, especially with multipathing. multipath occasionally can become sluggish when processing events, so the same might happen with it. We've tried to work around this, but never found a fool-proof way of doing so).

Adding Kay as he might have some more insight here.

Another thing:
Do you run LVM on top of multipathing?
If so, could you setup your system with _not_ using LVM and disabling the LVM service?

Reasoning here is that multipath should not be that susceptible to changes here than LVM2 is (don't nail me on this, I not _that_ into LVM2 details).

And as the system is stuck while waiting for LVM it might indeed be an side-effect when running LVM on top of multipathing.

Cheers,

Hannes
--
Dr. Hannes Reinecke		      zSeries & Storage
hare suse de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)


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